Field of the Invention
The present invention relates to a method for producing a barium titanate-based powder, and a multilayer ceramic capacitor.
Description of the Related Art
In a multilayer ceramic capacitor, an external electrode is formed at each of both end surfaces of a ceramic sintered body obtained by alternately laminating dielectric ceramic layers and internal electrode layers, and performing a firing treatment.
Since such a structure ensures that a large capacitance can be achieved while the size is kept small, multilayer ceramic capacitors are widely used with an increase in demand for mobile devices, etc., in recent years.
Multilayer ceramic capacitors are required to have a further reduced size and increased capacitance. For satisfying these requirements, thinning of the dielectric ceramic layers that form multilayer ceramic capacitors is promoted, but when dielectric ceramic layers are thinned, an electric field intensity applied per layer becomes relatively high. Accordingly, the reliability of the multilayer ceramic capacitor is easily deteriorated due to a small variation in grain size and reduction of crystallinity of ceramic grains that form the dielectric ceramic layer. Thus, it is required to improve reliability during application of a voltage, particularly life characteristics in a high temperature load test, by accurately controlling the grain size and crystallinity of ceramic grains that form the dielectric ceramic layer.
Japanese Patent Laid-open Publication No. 2009-190912 describes a method in which a titanium dioxide powder having a surface chlorine content and internal chlorine content of 2000 ppm or less in total, a surface chlorine content of 120 ppm or less, a rutile content of 30% or less and a BET specific surface area of 30 m2/g or more is provided, and a titanium dioxide powder is used to produce a barium titanate-based powder.
According to Japanese Patent Laid-open Publication No. 2009-190912, grain growth during production of barium titanate is suppressed, so that a barium titanate-based powder which is fine and has uniform grain properties and high tetragonality is obtained.
Japanese Patent Laid-open Publication No. 2009-190912 indicates that grain growth of titanium dioxide and barium titanate by residual chlorine is induced principally by surface chlorine of titanium dioxide grains. Specifically, chlorine ions (Cl groups) as an impurity are bound to the surfaces of titanium dioxide grains, and during production of barium titanate, uniformly distributed titanium dioxide grains are bound together with the surface chlorine serving as a core, so that abnormal growth is induced.
By adjusting the surface chlorine content of the titanium dioxide powder to 120 ppm or less Japanese Patent Laid-open Publication No. 2009-190912, abnormal growth is alleged to be suppressed.
Here, during production of a barium titanate-based powder, a step of adding water to a barium compound and a titanium compound to prepare a slurry, and performing wet-mixing is often employed.
Normally, BaCO3 is used as a barium compound, and TiO2 is used as a titanium compound.
The present inventors have found that the following event occurs in a process for production of a barium titanate-based powder, which includes a wet-mixing step.
When a slurry is prepared, a very small amount of BaCO3 as the barium compound is dissolved in water to generate Ba2+ ions. When Ba2+ ions exist in the slurry, at the time of drying the slurry, the Ba2+ ions react with CO2 in the air to be reprecipitated in the dry powder as BaCO3. The reprecipitated BaCO3 causes abnormal growth with itself serving as a core to increase variations in grain size, and also reduces the crystallinity of barium titanate.
That is, the present inventors have found that reprecipitation of BaCO3 leads to variations in grain size of the barium titanate-based powder and reduction of crystallinity of barium titanate, and hence deterioration of reliability as a multilayer ceramic capacitor.
It is believed that the invention described in Japanese Patent Laid-open Publication No. 2009-190912 is not effective against abnormal growth and reduction of crystallinity due to the above-mentioned mechanism.
In the invention described in Japanese Patent Laid-open Publication No. 2009-190912, reliability as a multilayer ceramic capacitor may be deteriorated because chlorine ultimately remains in a temporarily fired raw material.
The present invention has been devised for solving the problems described above, and an object of the present invention is to provide a method for producing a barium titanate-based powder while preventing reprecipitation of BaCO3 so as to suppress variations in grain size of the barium titanate-based powder and reduction of the crystallinity of barium titanate, so that a high-reliability multilayer ceramic capacitor can be produced.